This invention relates to packages for microelectronic frequency selection components such as surface acoustic wave (SAW) devices and more specifically to housings suitable for mass assembly of packaged hermetically sealed devices.
At the present time, much effort and expense is devoted to research and development relating to microelectronic frequency selection devices. Examples of devices which fall into this category are film bulk acoustic wave resonators (FBARs) (see, for example, "SBAR filter monolithically integrated with HBT amplifier," by D. Cushman et al., presented at the 1990 IEEE Ultrason. Symp., Honolulu, Hi., December 1990) and surface acoustic wave devices. Inasmuch as many such devices derive their electrical properties from mechanical motion of some portion of the device, events which affect that motion can in turn affect the frequency selection characteristics of such devices. Circuits including this type of device tend to provide fairly compact frequency selection components while taking up minimal space. However, the volume reduction achieved by improved packaging techniques can be as much as 100:1 as compared to other device packaging methods. These devices are not usable for practical applications unless they can be enclosed in and protected by proper hermetic packages or housings.
Prior art packages for such device chips tend to be expensive and tend to be much larger than the device enclosed within. While some prior art packages allow mounting of die within the package in a small area, the packages themselves require excessive amounts of space. This is because some such packages utilize leads that extend beyond the boundaries of the package. These leads often must be tied down to conductors on a substrate surface and then brought from the conductors on the substrate to an adjacent package. This requires that sufficient space be maintained between the prior art packages to facilitate the required electrical and mechanical connections.
Prior art die bonding by use of thermosetting polymers can result in trapping of contaminants in the polymer or in substantial outgassing of reaction products from the polymer on heating. These contaminants result in greatly impaired device performance when they condense on the frequency selection component surface, by promoting attenuation of the mechanical vibrations and thereby greatly diminishing the magnitude of the desired output signals.
For example, epoxies outgas large proportions of their mass during cure. Three leading brand adhesives tested by John Hopkins University scientists (Proc. IEEE Elec. Comp. Conf., IEEE Catalogue No. 89CH2775-5, pp. 301-308), outgassed 9 to 36% of their mass during cure. Such significant outgassing can result in void formation, diminishing the strength of the die bond, and can also result in re-deposition of heavier organic chemicals on the frequency selection component surface.
An additional problem occurs when automated machine placement and assembly of such components is attempted. This is due to bending of the leads during insertion into appropriately treated holes in the substrate, causing unreliable connections of the part to the surrounding circuitry.
Moreover, frequency selection components require hermetically sealed environments in order to work reliably. This has led to use of metallic packages, such as TO-5 and TO-8 containers, and various types of dual-in-line packages, which are individually resistance or seam welded. Due to the piezoelectric nature of the materials required for many types of frequency selection devices, such as surface wave filters, surface skimming bulk wave filters, shallow bulk acoustic wave filters, and the like, these devices must not be exposed to temperatures above the substrate Curie temperature. Such welding techniques produce adequately sealed hermetic packages without exposing the die to excessive heat. However, these manufacturing techniques are not well suited to batch processing, resulting in high costs which are prohibitive for high volume consumer product manufacturing.
Further, SAW die require mounting techniques and materials which do not result in stresses in the mounted die. Such stresses cause random phase distortion and frequency response distortion in the finished part.